Dual band framing reconnaissance camera

ABSTRACT

A framing aerial reconnaissance camera is described which has a Cassegrain optical system forming an objective lens that directs radiation to a spectrum-dividing prism. The prism directs radiation in the visible portion of the electromagnetic spectrum into a first optical path having a two-dimensional image-recording medium, such as a framing CCD array. Radiation in the infrared (IR) band of the spectrum is directed to a second optical path, which has a two-dimensional framing IR-sensitive image-recording medium. The entire camera can be either rotated about the aircraft roll axis in a continuous fashion or stepped in a series of steps to generate frames of imagery, providing panoramic coverage of the scene across the line of flight in two bands of the spectrum simultaneously.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following patent applications filedon the same date as this application, the contents of each of which isincorporated by reference herein:

Andrew J. Partynski et al., METHOD FOR FRAMING RECONNAISSANCE WITH ROLLMOTION COMPENSATION, Ser. No. 09/654,031;

Stephen R. Beran et al., METHOD OF FORWARD MOTION COMPENSATION IN ANAERIAL RECONNAISSANCE CAMERA, Ser. No. 09/652,965;

Stephen R. Beran et al., CASSEGRAIN OPTICAL SYSTEM FOR FRAMING AERIALRECONNAISSANCE CAMERA, Ser. No. 09/652,529.

BACKGROUND OF THE INVENTION

A. Field of the Invention

This invention relates generally to the field of aerial reconnaissancephotography and camera systems used for such photography. Moreparticularly, in a principal aspect the invention relates to areconnaissance camera that generates frames of imagery of terrain indifferent portions or bands of the electromagnetic spectrumsimultaneously.

The invention also relates to a novel method by which a cameracompensates for image motion due to both camera rotation and forwardmotion of the aircraft in which the camera is installed. Such imagemotion compensation allows for high-resolution images to be obtainedfrom the camera system.

B. Description of Related Art

Long Range Oblique Photography (LOROP) cameras have been developed as aresult of the need to obtain clear, high resolution pictures from longerranges, typically from 10 to 50 nautical miles from the terrain ofinterest. The advent of LOROP cameras was an outgrowth of development ofweapons technology, which could engage reconnaissance aircraft atever-increasing distances, and geopolitical boundaries that became moreand more difficult to encroach upon.

With the advent of LOROP cameras came the operational intricacies ofusing very sensitive and high performance instruments in a fashion thatyielded the intelligence, i.e., image resolution, required of them.These operational issues were hostage to the technological limitationsof the day. Initially, all cameras were film. Film LOROP cameras havebeen operated both as panoramic scanning (line scan) and framingcameras. Panoramic scan cameras collect an image with a smooth rollingmotion of the camera while exposing film by pulling it passed a slit.The advantage of this approach was ease of implementation of thescanning mechanism. The disadvantage is that each line of exposedimagery was taken from a different perspective, hence the scanningsystem inherently was prone to creating geometrically and geospatiallydistorted images.

Subsequently, LOROP film framing cameras were employed. These camerascaptured a frame of imagery by rapidly moving a slit across the film forexposure. The cameras utilized a scan head mirror assembly that could bemoved in order to take successive frames of imagery at a selecteddepression angle relative to the horizon, depending on the targetlocation.

Later, electro-optical line scan cameras entered the market as afilmless solution. Instead of film, the cameras used a solid statelinear line scan charge coupled device (CCD) as a detector. Thesecameras used a scan mirror or the motion of the ground below theaircraft to scan the image across the line of photosensitive detectorsthat made up the CCD to form a frame, line by line. Again, thedisadvantage of this method was that imagery was obtained from adifferent perspective as the aircraft moved, resulting in geometricallyand geospatially distorted images.

Step framing cameras were developed which take a full frame of imageryat one time, then step the camera to a new angular position, take thenext frame of imagery (with some overlap between the images to insure100% coverage), step and generate a new frame of imagery, and so onuntil the desired scene is covered. The disadvantage of step framingcameras was that the stepping action was very difficult to accomplishwith the whole camera, therefore it had to be broken into a scan headthat performed the stepping and an image de-rotation mechanism, both ofwhich were tied together by a synchronized drive system. The advantagesof step frame cameras as compared to line scanning cameras are highergeometric fidelity and geo-spatial accuracy. Originally, full framingcameras were all film.

The next revolutionary step in the art of LOROP and tactical aerialreconnaissance cameras was the development of two-dimensional area arrayelectro-optical (E-O) detectors. This occurred several years after theelectro-optical linear arrays were first developed, and requiredsemiconductor processing technology to mature many more years beforesuch arrays were practical for reconnaissance use. Recon/Optical, Inc.,the assignee of the present invention, in the early 1990's, introducedlarge area focal plane arrays to the reconnaissance industry. One sucharray is described in U.S. Pat. No. 5,155,597 to Andre G. Lareau et al.,the contents of which are incorporated by reference herein. Such cameraswere the first large area arrays to be used in tactical aircraft, aswell as strategic reconnaissance aircraft such as the high altitudeSR-71 aircraft. These large area arrays had the advantage of providingan image from a single point in space giving excellent geometricfidelity. Moreover, the high pixel count, and optimal pixel size,allowed such cameras to produce imagery having outstanding imageresolution.

Furthermore, as described in the '597 Lareau et al. patent, it waspossible to perform forward motion compensation in side oblique, forwardoblique and nadir camera orientations electronically. U.S. Pat. No.5,668,593, also to Lareau et al., describes a step-frame electro-opticcamera system with electronic forward motion compensation. U.S. Pat. No.5,798,786, also to Lareau et al., describes a method for compensationfor roll, pitch or yaw motions of an aerial reconnaissance vehicle, inaddition to forward motion compensation, electronically in the focalplane of an E-O detector. The '593 and '786 Lareau et al. patents areincorporated by reference herein.

Framing E-O LOROP camera systems were a logical platform to host theadvanced detectors such as described in the Lareau et al. '597 patent.Electro-optical detectors, such as described in the Lareau et al. '597patent, are capable of being fabricated from selected materials that candetect incident radiation in a variety of portions of theelectromagnetic spectrum, and not just the visible spectrum. Inparticular, the advantages of large area framing can be enhanced byproviding imaging capability in the infrared (IR) portion of thespectrum. A camera that generates frames of imagery in two distinctportions of the electromagnetic spectrum simultaneously is referred toherein as a “dual band framing camera.” The patent to Gilbert W. Willey,U.S. Pat. No. 5,841,574, also assigned to Recon/Optical, Inc., describesa multi-spectral, decentered aperture, catadioptric optical systemparticularly suitable for a dual band line scanning camera system havingtwo linear electro-optical detectors, one for the visible or near IR(λ=0.5 to about 1.0 microns), and one for either the mid-wavelength IR(λ=about 3.0 to about 5.0 microns) or the long-wavelength IR (λ=about8.0 to about 14.0 microns).

The technological capability of dual band framing LOROP cameras promisesperformance heretofore unavailable anywhere. However, the implementationof such a camera presents a number of difficulties and technicalchallenges beyond those posed for prior art systems. These challengesare optical, servo-mechanical and operational, and are discussed infurther detail below. The present invention provides a dual band framingaerial reconnaissance camera system that overcomes these challenges anddifficulties to provide an advanced, high resolution framing camerasystem that generates imagery of a scene of interest at two differentbands of the electromagnetic spectrum.

SUMMARY OF THE INVENTION

A dual-band framing aerial reconnaissance camera for installation in anaerial reconnaissance vehicle has been invented. The camera includes anoptical system incorporated into a camera housing. The optical systemcomprises an objective optical subassembly that receives incidentradiation from a scene external of the vehicle. Radiation from the sceneis reflected from the objective optical subassembly to aspectrum-dividing prism. The prism directs radiation in a first band ofthe electromagnetic spectrum, such as visible and near IR, into a firstoptical path and directs radiation in a second band of theelectromagnetic spectrum, such as midwavelength IR or long wavelengthIR, into a second optical path different from the first optical path.The first optical path includes suitable image forming and focusinglenses and a first two-dimensional image-recording medium for generatingframes of imagery in the first band of the electromagnetic spectrum. Thesecond optical path also includes suitable image forming and focusinglenses and a second two-dimensional image-recording medium generatingframes of imagery in the second band of the electromagnetic spectrum.

The camera further includes a servo-mechanical subsystem. This subsystemincludes a first motor system coupled to the camera housing that rotatesthe entire camera housing (including the optical system as recitedabove) about a first axis. The camera housing is installed in the aerialreconnaissance vehicle such that this first axis of rotation is parallelto the roll axis of the aerial reconnaissance vehicle (referred toherein for simplicity as “the roll axis”). The image recording media areexposed to the scene to generate frames of imagery as the first motorsystem rotates the camera housing in a continuous fashion about the rollaxis. The first and second image recording media have a means forcompensating for image motion due to the rotation of the camera housing.In an electro-optical embodiment of the image recording media, the rollmotion compensation means is preferably comprised of electroniccircuitry for clocking or transferring pixel information through theelectro-optical detectors at a uniform rate substantially equal to therate of image motion due to camera rotation. A method of calculating theimage motion rate, and thus pixel information transfer rate, due to rollof the camera housing is disclosed herein. If a film camera is used forthe image recording media, a mechanical system is used to move the filmat a rate substantially equal to the image motion rate.

The servo-mechanical subsystem also includes a second motor systemcoupled to the objective optical subassembly. In the illustratedembodiment, the objective optical subassembly comprises a catoptricCassegrain optical system. The second motor system rotates theCassegrain optical system about a second axis in the direction offorward motion of the reconnaissance Vehicle to compensate for theforward motion of the aerial reconnaissance vehicle. The action of thefirst motor assembly to rotate the entire camera housing about the rollaxis occurs at the same time (i.e., simultaneously with) the action ofthe second motor system to rotate the Cassegrain optical system in theline of flight to accomplish forward motion compensation. The net effectof the action of the Cassegrain motor system and the roll motioncompensation system is that the image of the scene of interest isessentially frozen relative to the focal plane of the image recordingmedia while the media obtain the frames of imagery, allowing highresolution images of the scene in two different bands of the spectrum tobe obtained simultaneously. Furthermore, the rotation of the image scenecaused by the roll motion of the objective subassembly is simultaneouslydetrotated by the roll motion of the rest of the camera, in view of thefact that the entire camera assembly is rolled as a unit, therebyeliminating the need for a separate derotation mechanism such as apechan prism. Other types of optical arrangements for the objectiveoptical subassembly are possible, but are less preferred. The operationof the camera with the different type of objective subassembly is thesame.

In a preferred embodiment, the first and second image recording mediacomprise two dimensional area array electro-optical detectors. One maybe manufactured from materials sensitive to radiation in the visible andnear-IR portion of the electromagnetic spectrum, and in a preferredembodiment is a charge-coupled device (CCD) detector of say 5,000×5,000pixels. The other of the electro-optical detectors is made from amaterial sensitive to radiation in the infrared portion of theelectromagnetic spectrum, and may be a platinum silicide array of photodiode detectors or other suitable type of electro-optical detectorsuitable for IR detection. The reader is directed to U.S. Pat. No.5,925,883 to Woolaway, III, the contents of which are incorporated byreference herein, for a description of an IR detector. The detectorsensitive to radiation in the infrared portion of the electromagneticspectrum is preferably sensitive to radiation having a wavelength ofbetween 1.0 and 2.0 microns (SWIR), 3.0 and 5.0 microns (MWIR), or fromabout 8.0 to about 14.0 microns (LWIR). In either of the embodiment ofelectro-optical detectors, they will typically comprise an array ofpixel elements arranged in a plurality of rows and columns. The meansfor compensation for roll motion of the camera housing compriseselectronic circuitry for transferring pixel information in theelectro-optical detectors from row to adjacent row at a pixelinformation transfer rate (uniform across the array) substantially equalto the rate of image motion in the plane of the electro-opticaldetectors due to roll of the camera housing. The transfer of pixelinformation occurs while the pixel elements are integrating chargerepresenting scene information. Thus, the roll motion compensation canbe performed electronically on-chip.

As a further possible embodiment, electro-optical detectors with thecapability for transferring pixel information in both row and columndirections independently, such as described in Lareau et al., U.S. Pat.No. 5,798,786, could be used for the image recording media. Forwardmotion compensation and roll motion compensation could be performedon-chip in the detectors.

As noted above, the present invention required the solution to severaldifficult technical challenges, including optical, servo-mechanical andoperational difficulties. For an electro-optical framing LOROP camera tooperate in at least two discrete bands of the electromagnetic spectrumat the same time, the optical challenge is to focus panchromatic energy(e.g. visible through IR) on a focal plane detector with (1) good imagequality and satisfactory modulation transfer function, (2) whilebaffling stray energy, (3) meeting space constraints, and (4) enablingthe use of a relatively large two-dimensional area array as a focalplane detector to get an adequate field of view and resolution. Inaccordance with one aspect of the invention, these optical challengeswere solved by a unique catoptric Cassegrain objective opticalsubassembly incorporating an azimuth mirror and utilizing separate fieldoptics for each band of the spectrum, described in more detail herein.

The catoptric Cassegrain type of objective optical subassembly does notlend itself to the use of servo-mechanical systems developed for priorart LOROP systems, particularly those used in prior art step framecameras (such as described in the Lareau et al. '593 patent). The priorart step frame cameras use a stepping mirror to step across the line offlight and direct radiation onto the array, and require a de-rotationmechanism, such as a Pechan prism, to de-rotate the images. The standardsolution of stepping the entire LOROP camera system or even a large scanmirror assembly at the operational frame rate are not acceptablealternatives for large LOROP cameras, an in particular large dual bandsystems. In particular, the applications of the present invention areflexible enough to include both strategic and tactical aircraft, as wellas the new breed of aircraft being used by the military forreconnaissance known as unmanned aerial vehicles (including lowobservables). The diversity of these applications posed a power andstability problem that prevents application of prior art solutions. Thetask of stepping a 400 lb. camera mass two to four times a secondcreates tremendous inertial loads as well as power spikes that would beunacceptable. Even the inertia and associated settling times of astepped scan head assembly pose problems in some applications.

This servo-mechanical situation required a unique inventive solution,described in detail herein. The solution, as provided in one aspect ofthe present invention, was to (1) rotate the entire camera (includingthe entire optical system and the image recording media) smoothly in acontinuous fashion about an axis parallel to the aircraft roll axis,similar to the pan-type movement, but without the starts and stops usedin a traditional step-frame camera system, and (2) operating the cameraas a framing camera while the camera undergoes the smooth rotation.Frames of imagery are thus taken while the camera smoothly rotates aboutthe roll axis at a constant angular velocity. In addition to this novel“roll-framing” technique, the present invention also electronicallycompensates for, i.e., stops, the image motion due to roll while thecamera is scanning in a smooth motion. Meanwhile, a novel forward motioncompensation technique is performed by the Cassegrain opticalsubassembly to cancel out image motion effects due to the forward motionof the aircraft. The result enables exposures of the image recordingmedia to the scene while compensating for roll and forward motion,enabling high-resolution images to be obtained.

The present invention thus solves the difficult optical,servo-mechanical and operational problems and provides a dual bandframing electro-optical LOROP camera that delivers a performance andtechnical capability that has never before been achieved. In particular,it provides a system by which high-resolution frames of imagery in twodifferent portions of the electromagnetic spectrum can be generatedsimultaneously. The inventive camera can be used in a quasi-steppingmode, in which overlapping frames of imagery are obtained across theline of flight. It can also be used in a spot mode, in which the camerais oriented in a particular direction to take an image of a targetexpected to be in the field of view.

Many of the teachings of the present invention are particularlyapplicable to a dual band electro-optical framing reconnaissance camera,and such a camera is the preferred embodiment. However, as explainedbelow, some of the techniques and methods of the subject camera system,such as the roll-framing operation and unique roll and forward motioncompensation techniques, are applicable to a camera system that imagesterrain in only one portion of the electromagnetic spectrum. Thus, in analternative embodiment the camera is basically as set forth as describedabove, except that only a single detector is used and thespectrum-dividing prism and second optical path are not needed.Furthermore, while a preferred embodiment uses a two-dimensionalelectro-optical imaging array for the detector in each of the bands ofthe electromagnetic spectrum, the inventive camera system can be adaptedto use film or other types of detectors for the photosensitive recordingmedium. In the film camera embodiment, roll motion compensation could beperformed by moving the film in a manner such that the film velocitysubstantially matches the image velocity due to roll.

While the foregoing summary has described some of the highlights of theinventive camera system, further details on these and other featureswill be described in the following detailed description of a presentlypreferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Presently preferred embodiments of the invention will be discussed belowin conjunction with the appended drawing figures, wherein like referencenumerals refer to like elements in the various views, and wherein:

FIG. 1 is a perspective view of an aircraft flying over a terrain ofinterest with a camera in accordance with the preferred embodimentoperating to generate frames of imagery of the terrain in two bands ofthe electromagnetic spectrum simultaneously.

FIG. 2 is a schematic representation of the aircraft of FIG. 1 taking aseries of 5 frames of images in a series of cycles while flying past theterrain of interest;

FIGS. 2A and 2B are perspective view of the camera system of FIG. 1,shown isolated from the rest of the aircraft, and with protective coversremoved in order to better illustrate the components of the camera;

FIG. 2C is a perspective view of the camera of FIGS. 2A and 2B, with theprotective covers installed, and showing the entrance aperture for thecatoptric Cassegrain optical system;

FIG. 3 is a top plan view of a presently preferred embodiment of thedual band framing reconnaissance camera system of FIGS. 2A-2C, with thecovers removed;

FIG. 4 is a cross-sectional view of the camera system of FIG. 3, takenalong the lines 4—4 of FIG. 3;

FIG. 4A is a simplified ray diagram of the optical system of FIGS. 3 and4;

FIGS. 4B and C are more detailed cross-sectional views of the opticalelements in the visible and MWIR paths of FIGS. 4 and 4A;

FIG. 5 is an end view of the camera system of FIGS. 3-4, shown from theright-hand end of the camera housing and with the roll motor and coverplate at that end removed in order to better illustrate the otherstructures in the camera;

FIG. 6 is a perspective view of the assembly of the Cassegrainsubsystem, showing in better detail the structure that retains theCassegrain primary mirror and showing the secondary mirror, azimuthmirror, Cassegrain motor assembly and azimuth 2-1 drive assembly ingreater detail. The primary mirror itself is removed from FIG. 6 inorder to better illustrate the components of the Cassegrain opticalsystem.

FIG. 7 is another perspective view of the Cassegrain primary mirrorretaining assembly of FIG. 6;

FIG. 8 is another perspective view of the Cassegrain primary mirrorretaining assembly as seen generally from the rear, shown partially insection;

FIG. 9 is a top view of the Cassegrain optical system of FIG. 6;

FIG. 10 is a cross-sectional view of the Cassegrain optical system ofFIG. 6, taken along the line 10—10 of FIG. 9;

FIG. 11 is a detailed sectional view of the azimuth mirror 2-1 driveassembly that rotates the azimuth mirror at one half the rate ofrotation of the entire Cassegrain optical subsystem;

FIG. 12 is a detailed perspective view of one of the roll motorassemblies of FIG. 2, showing the L shaped brackets that mount to thestator of the motor and rigidly couple the roll motor to the pod oraircraft;

FIG. 13 is an elevational view of the roll motor of FIG. 12;

FIG. 14 is a cross-sectional view of the roll motor of FIG. 14;

FIG. 15 is a detailed illustration of a portion of the roll motor ofFIG. 14;

FIG. 16A is a ray diagram of the visible path in the embodiment of FIG.4;

FIG. 16B is a ray diagram of the MWIR path in the embodiment of FIG. 4;

FIG. 16C is a graph of the visible path diffraction modulation transferfunction;

FIG. 17 is a block diagram of the electronics for the camera system ofFIGS. 2-5;

FIG. 18 is schematic representation on an image recording medium in theform of a two dimensional electro-optical array, showing the imagemotion in the array due to the roll of the camera; and

FIG. 19 is another schematic representation of the array of FIG. 18,showing the electronic circuitry that controls the transfer of pixelinformation in the array at the same velocity as the image in order toprovide roll motion compensation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Overview and Method of Operation

Referring now to FIG. 1, an aerial reconnaissance camera system 20 inaccordance with a preferred embodiment of the invention is showninstalled in a reconnaissance aircraft 22 flying over a terrain ofinterest 42 at an altitude H and with forward velocity V, moving in adirection of flight FL. The aerial reconnaissance camera system 20includes a camera 36, shown in greater detail in FIGS. 2-2C and 3-5, acamera control computer 34 and associated electronics described infurther detail in FIGS. 17 and 19. The camera control computer receivescertain navigational information from the aircraft avionics system 24,including current aircraft velocity and height data. Additional camerasystem inputs may come from a console 28 in the cockpit, such as startand stop commands or camera depression (roll angle) settings.

The aircraft body defines a roll axis R, a pitch axis PI and a yaw axisY passing through the center of gravity CG of the aircraft. The camera36 is shown orientated at a camera depression angle δ relative to abilateral plane BP that is horizontal during level flight. In theillustrated embodiment, the line of sight LOS of the camera 36 isnominally orthogonal to the roll axis in a side oblique or nadirorientation.

The preferred embodiment of the subject camera system 20 operates like astep-frame electro-optic (E-O) sensor, capable of taking a sequence ofoverlapped frames in the cross-track, i.e., cross-line of flight,direction. This is shown in FIG. 2. As the aircraft flies by the terrainof interest, the camera is rotated about the roll axis in a continuousfashion (i.e., without starts and stops between frames), with frames ofimagery taken at different depression (roll) angles, e.g., angles δ1,δ2, δ3, δ4 and δ5, resulting in frames 1, 2, 3, 4 and 5. A nominal rateof rotation about the roll axis is used (based on focal length, arrayframe size and framing rate, such as 8-10 degrees per second, but theroll rate is adjustable by the camera control computer. When the fifthframe of imagery is obtained and the camera rolled to its roll limitposition (either pre-set or commanded by the operator), the camerarotates back, i.e., retraces, to its initial roll position (δ1), and thesecond cycle of frames of imagery is obtained (1A, 2A, 3A, 4A, 5A). Theprocess repeats for a third and subsequent cycles of operation.

The cross-track framing sequence 1, 2, 3, 4, 5; 1A, 2A, 3A, 4A, 5A; etc.(which is V/H dependent) can be made in either spectrum individually orin both spectrums simultaneously, dependent on the time of day and thepurpose of the reconnaissance mission. As noted in FIG. 2, the rollaction of the camera can encompass both sides of nadir, for example withframes 1-4 obtained at one side of nadir and frame 5 obtained at theother side of nadir. The camera can also be used in a spot mode, inwhich the camera is rotated to a particular depression angle and framesof imagery obtained of the scene of interest. The number of frames percycle of roll, N, can thus vary from 1 to say 5 or 10 or until horizonto horizon coverage is obtained.

FIG. 2A shows the camera 36 in a perspective view as seen from below,with a set of protective cover plates removed in order to betterillustrate the structure of the camera. FIG. 2B is another perspectiveview, shown from above, and FIG. 2C is a perspective view of the camera36 with the cover plates 33 installed, showing the entrance pupil 35 forthe camera. Referring now to FIGS. 3-5, the camera 36 per se is shown intop, sectional and end views, respectively. In the end view of FIG. 5, arear support plate 41 and a roll motor 70A are removed in order tobetter illustrate the rest of the camera 36.

As shown best in FIG. 3, the camera 36 mounts to the reconnaissance podor airframe of the aerial reconnaissance vehicle via four mountingbrackets 39, each connected to the pod or airframe via passive shockisolation mounts in conventional fashion. The mounting brackets 39 arebolted to the sides of the stator of the roll motor assemblies 70A and70B as shown in FIG. 12 and described below. The entire camera cylindercomprising all the components between the two support plates 41 and 41Acan rotate relative to the roll axis 37 while the stator of the rollmotors 70A and 70B and mounting brackets 39 remain in a fixed positionrelative to the aerial reconnaissance vehicle.

The basic configuration of the camera 36 is a cylinder, as perhaps bestillustrated in FIG. 2C, which in the illustrated embodiment isapproximately 20 inches in diameter and 48 inches in length. The camera36 is installed in an aircraft reconnaissance pod via the mountingbrackets 39 such that the cylinder axis 37 is oriented nominallyparallel to the flight direction of the aircraft, i.e., the roll axis ofthe aircraft. The fore/aft orientation of the camera can be either way.Additionally, the camera 36 can be installed such that it is orientedperpendicular to the line of flight.

A typical use of the camera is to take overlapping frames of images inthe cross-track direction as the aircraft flies over the scene ofinterest as shown in FIG. 2, similar in concept to the step frameoperation described in the prior art patent of Lareau, et al. U.S. Pat.No. 5,668,593 and earlier step frame film cameras. However, the mannerin which the camera achieves this result is very different from thattaught in the prior art. Whereas in the Lareau '593 patent, a steppingmirror is rotated in discrete steps to image the terrain, and forwardmotion compensation is performed in the array itself electronically, inthe preferred embodiment of the present invention the entire camera 36is rotated at a constant angular velocity, and in a continuous fashion,about the roll axis 37. The roll rate is determined by the opticalsystem focal length, frame size, frame rate and the desired cross-trackoverlap (typically 5%) between consecutive frames. Moreover, forwardmotion compensation is achieved by means of rotation of the Cassegrainoptical system about an axis 75, as described below, not in the array.

Referring to FIGS. 3 and 4, the camera includes an optical system 50which is incorporated into (i.e., mounted to) a camera housing orsuperstructure 52. The optical system 50 in the preferred embodimentcomprises a novel catoptric Cassegrain objective optical subassembly 54which receives incident radiation from a scene external of the vehicle.FIG. 4A shows a simplified ray diagram for the optical system 50. TheCassegrain objective optical subassembly includes a primary mirror 80, asecondary mirror 82 and a flat azimuth mirror 84. The secondary mirror82 is centrally located in the entrance aperture of the Cassegrainoptical system. Radiation from the scene is reflected from theCassegrain objective subassembly 54 to a spectrum-dividing prism 56. Theprism 56 directs radiation in a first band of the electromagneticspectrum, such as visible and near IR, into a first optical path 58 anddirects radiation in a second band of the electromagnetic spectrum, suchas mid-wavelength IR or long wavelength IR, into a second optical path60 different from the first optical path. The first optical path 58includes suitable image forming and focusing lenses 62 and a firsttwo-dimensional image recording medium 64 for generating frames ofimagery in the first band of the electromagnetic spectrum. The secondoptical path 60 includes a fold prism 61, suitable image forming andfocusing lenses 66 and a second two-dimensional image recording medium68 which generates frames of imagery in the second band of theelectromagnetic spectrum.

The camera further includes a novel servo-mechanical subsystem. Thissubsystem includes a first motor system 70A and 70B coupled to thecamera housing 52 that rotates the camera housing 52 (including theoptical system 50 as recited above) about the roll axis 37. The imagerecording media 64 and 68 are exposed to the scene to generate frames ofimagery as the first motor system 70A and 70B rotates the camera housing52 in a continuous fashion about the roll axis 37. The first and secondimage recording media have a means for compensating for image motion dueto the rotation of the camera housing. In an electro-optical embodimentof the image recording media, the roll motion compensation means ispreferably comprised of electronic circuitry for clocking ortransferring pixel information through the electro-optical detectors ata uniform rate substantially equal to the rate of image motion due tocamera rotation. A method of calculating the image motion rate, and thuspixel information transfer rate, due to roll of the camera housing isdescribed below. If a film camera is used for the image recording media,a mechanical system is used to move the film at a rate substantiallyequal to the image motion rate. Film drive mechanisms for moving filmfor purposes of motion compensation are known in the art and can beadapted for a film framing camera for purposes of roll motioncompensation by persons skilled in the art.

The servo-mechanical subsystem also includes a second motor system 74,shown best in FIGS. 3, 5 and 6, coupled to the front end of theCassegrain optical system 54. The second motor system 74 rotates theCassegrain objective subassembly 54, including the primary, secondaryand azimuth mirrors, about a second axis 75 in the direction of forwardmotion of the reconnaissance vehicle in a manner to compensate forforward motion of the aerial reconnaissance vehicle. In the illustratedembodiment, the azimuth mirror 84 is rotated about the axis 75 at onehalf the rate of rotation of the Cassegrain primary and secondarymirrors 80 and 82 in the direction of forward motion. The action of thefirst motor assembly 70A and 70B to rotate the entire camera housingabout the roll axis occurs at the same time (i.e., simultaneously with)the action of the second motor system 74 to rotate the Cassegrainoptical system 80, 82 and 84 in the line of flight to accomplish forwardmotion compensation. The net effect of the action of the Cassegrainmotor system 74 and the roll motion compensation technique is that theimage of the scene of interest is essentially frozen relative to thefocal plane of the image recording media while the image recording mediaobtain the frames of imagery, allowing high resolution images of thescene in two different bands of the spectrum to be obtainedsimultaneously.

During operation, as the entire camera 36 rotates by action of the rollmotors 70A and 70B, exposure of the detectors 64 and 68 at the two focalplanes is made. In the illustrated embodiment, in the visible spectrumpath 58 the exposure is executed by means of a mechanical focal planeshutter 88 which opens to allow incident photons to impinge on atwo-dimensional charge-coupled device E-O detector array 64. In the MWIRpath 60, exposure is executed by electronic switching (on/off) ofIR-sensitive photocells arranged in a two-dimensional array 68,basically by dumping charge accumulating prior to the initiation ofexposure and then accumulating and storing charge when the exposureperiod commences. However, any method of exposure control will work withthis roll-framing camera.

When the initial exposure is complete, the data is read out from the twofocal plane detectors 64 and 68 and they are placed in condition for asecond exposure. The rotation of the entire camera assembly about theroll axis 37 continues smoothly (that is, without starting and stoppingas for example found in a prior art step frame camera system). When thenext exposure is ready to be taken, i.e., when depression angle δ2 ofFIG. 2 has been reached, the shutter is opened in the visible/near IRpath; similarly, in the MWIR path the charge dumping ceases and chargeis accumulated. The data is then read out of the two focal plane sensorsafter the exposure period is over. Meanwhile, the rotation of the entirecamera system about the roll axis continues without interruption and athird and subsequent exposure of both cameras is taken if time permits.The process continues until the angular limit of the framing cycle hasbeen reached, at which time the roll motors 70A and 70B retrace theirangular rotation and return to their original angular position. Theprocess then repeats for a new cycle of framing, as indicated in FIG. 2.

The camera system roll rate (the cylinder angular velocity), ω, isestablished as follows. First, determine the cross-track field of viewper frame, ω, according to equation (1):

Φ=2Arctan(W/2f),  (1)

where

W=detector array size in the cross-track direction; and

f=lens focal length (i.e, the focal length of the overall aggregate ofoptical components in the particular band of interest, e.g., the visibleband).

Then, the cylinder angular velocity ω is computed according to equation(2):

ω=Φ(FR)(1-OL _(c)),  (2)

where

FR=system frame rate (frames per second)

OL_(c)=overlap between consecutive cross-track frames (expressed as adecimal).

Note that the cylinder angular velocity ω is independent of theaircraft's velocity and height above the earth. Typical angularrotations between the successive exposures of the array will be lessthan 10 degrees.

Since the focal plane detectors are rotating about the roll axis duringthe exposure period, the scene image is translating across each of thedetector arrays in the cross-track direction at a fixed velocity v=fω.The image motion due to camera roll is constant and uniform across thearray. To compensate for this image motion, and thereby preserveresolution, this image motion is synchronized with the velocity at whichcharge representing scene information is transferred within the detectorarrays, thus eliminating relative motion between the image and thepixels imaging the scene and thus eliminating the image smear that wouldotherwise take place at the detector. In other words, pixel informationin the entire array is transferred in the direction of image motion fromrow to adjacent row at a rate that substantially matches the imagevelocity v.

At the end of the exposure period (typically 0.0005 to 0.020 seconds),the cylinder continues rotating to the next scene position while thecollected scene signals are read-out of the detector array(s). Note thatthere is no rotational start and stop between exposures, as found inprior art step frame camera systems, thereby avoiding the servo loopsettling times, load current surges, and power spikes produced bymechanical stepping systems as noted earlier.

In this “roll-framing” type of operation, the two focal plane detectors64 and 68 operate in the above manner, taking N consecutive cross-trackframes, N being dependent on the time available or by the intended mode(maximum coverage, limited coverage or spot mode) of operation. Theresult is a series of frames of images similar to that produced with astep frame camera system, as indicated in FIG. 2, each frame taken intwo different bands of the electromagnetic spectrum. In maximum coveragemode, N is determined by the V/H ratio of the mission, the camera systemdepression angle range and the framing rate, and N can be as many as 10frames/cycle (or more) in normal operation. At the end of thecross-track cycle, the camera system or cylinder is rotated back (reset)to the first frame angular position and the cycle repeats until theintended in-flight direction coverage is achieved. The camera cangenerate overlapping frames of imagery similar to that shown in FIG. 1,where N in the illustrated example=5.

In spot mode, a one or two frames/cycle is executed, with the cameraaimed at a specific predetermined depression (roll) angle and fore/aftazimuth angle where a target or specific interest is expected to be. Inthis example, N will typically equal 1 or 2. The cycle may repeat for asmany times as needed.

As another mode of operation, the camera could be used in a traditionalstep frame operation. In this mode, the camera would rotate betweensuccessive angular positions, and the photosensitive media wouldgenerate two-dimensional images of the terrain. If the camera bodyrotation is stopped during scene exposure, forward motion compensationcould be performed in the photosensitive media, such as described in theearlier Lareau et al. patents.

The preferred forward motion compensation method will now be describedwith a little more specificity. As the exposures are made at either ofthe two detectors 64 and 68, the aircraft is moving at some knownvelocity. The forward motion of the aircraft is neutralized in a novelway in the preferred embodiment. Whereas in the prior art Lareau et al.'586 patent forward motion compensation is performed on-chip in thearray, the forward motion compensation of the preferred embodiment isperformed by rotation of the Cassegrain objective subassembly, i.e, theCassegrain primary and secondary mirror assembly, in the flightdirection at a rate=V/R (in units of radians per second) where V is theaircraft velocity and R is the range to the scene of interest. The valueof R can be derived from simple geometry from the known aircraft heightand camera depression angle (δ_(i)) and assuming the earth is flat inthe scene of interest, from a Global Positioning System on board theaircraft, using an active range finder, or by computing range fromsuccessive frames of imagery as described in the patent of Lareau etal., U.S. Pat. No. 5,692,062, which is incorporated by reference herein.As the Cassegrain primary and secondary mirrors 80 and 82, respectively,are rotated at the V/R rate in the direction of flight, the flat azimuthmirror 84, located in the optical path between the secondary reflectorand the Cassegrain image plane 86, is rotated at a rate equal to ½ (V/R)in the same direction, thus “stopping” image motion due to aircraftforward motion at the image plane. Thus, the rotating Cassegrainobjective lens and the half speed azimuth mirror provide the neededforward motion compensation function.

As an alternative embodiment, the Cassegrain optical system could remainfixed and both forward motion compensation and roll motion compensationcould be performed in the focal plane detector by transferring pixelinformation in both row and column directions in accordance with theprinciples of the Lareau et al. patent, U.S. Pat. No. 5,798,786.

From the FIGS. 1-5 and 18 and the above discussion, it will beappreciated that we have invented a method of generating frames ofimagery of a scene of interest with an aerial reconnaissance camera intwo different bands of the electromagnetic spectrum simultaneously. Themethod includes the steps of:

(a) providing two photosensitive electro-optical detectors 64, 68 in thecamera 36, each of the detectors comprising an array of pixel elementsarranged in a plurality of rows and columns;

(b) rotating the camera 36 in a continuous fashion about a roll axis 37either coincident with or parallel to a roll axis R of an aerialreconnaissance vehicle carrying the camera;

(c) while rotating the camera 36, simultaneously exposing theelectro-optical detectors 64, 68 to a scene of interest in a series ofexposures;

(d) while rotating the camera 36 and while exposing the electro-opticaldetectors 64 and 68 to the scene, rotating an optical system 54providing an objective lens for the camera in the direction of forwardmotion of the vehicle at a predetermined rate to cancel out image motiondue to forward motion of the vehicle; and

(e) while the electro-optical detectors 64 and 68 are being exposed tothe scene, moving pixel information in the arrays at a rate and in adirection substantially equal to the rate of image motion due torotation of the camera about the roll axis, to thereby preserveresolution of images generated by the detectors.

Table 1 lists performance specifications for a presently preferred dualband step frame camera system in accordance with the invention.

TABLE 1 Focal Length & f/# Visible Channel 50.0 inches-f/4.0 (Options)72.0 inches-f/5.8 84.0 inches-f/6.7 MWIR Channel 50.0 inches-f/4.0Optical System Type: Cassegrain objective lens with spectrum beamdivider and individual visible channel and MWIR channel relay lenses.Operating Spectrums = Visible Channel −0.50to 0.90 microns MWIR Channel−3.0 to 5.0 microns Entrance Pupil Diameter: 12.5 inches, both channels,all focal lengths. Detectors: Visible Channel: 5040 × 5040 pixels .010mm × .010 mm pixel pitch 50.4 mm × 50.4 mm array size 4.0 frames/secmax. MWIR Channel: 2016 × 2016 pixels .025 mm × .025 pixel pitch 50.4 mm× 50.4 mm array size 4.0 frames/sec max. MWIR Channel: 2520 × 2520pixels (future) .020 mm × .020 mm pixel pitch 50.4 mm × 50.4 mm arraysize 4.0 frames/sec max. FOV (per frame): VIS Channel: 2.27° × 2.27° (50inch F.L.) 1.58° × 1.58° (72 inch F.L.) 1.35° × 1.35° (84 inch F.L.)MWIR Channel: 2.27° × 2.27° (50 inch F.L.) Frame Rates: Variable, up to4.0 fr/sec Both channels, aIl focal lengths. Pixel IFOV: VIS Channel:7.9 × 10⁻⁶ RAD (50 inch F.L.) 5.5 × 10⁻⁶ RAD (72 inch F.L.) 4.7 × 10⁻⁶RAD (84 inch F.L.) MWIR Channel: 19.7 × 10⁻⁶ RAD (50 inch F.L.) 15.8 ×10⁻⁶ RAD (50 inch F.L.) (future) Ground Resolvable Distance (GRD) (atrange, perpendicular to the LOS). VIS Channel: 3 ft @ 31 N mi. (NIIRS-5)(50″) 3 ft @ 45 N mi.(NIIRS-5) (72″) 3 ft @ 52 N mi. (NIIRS-5) (84″)MWIR Channel: 3 ft @ 12.5 N mi. (NIIRS-5) (50″) (Future) 3 ft @ 15.6 Nmi. (NIIRS-5) (50″) Field of Regard: Horizon to Horizon, or as limitedby vehicle windows (5° to 30° depression below horizon (δ) is typical).Scene coverage Variable cross-track. rate: Roll rate: 8.6°/sec-(50 inchfocal length, 4 Fr/Sec.)

Preferred Dual Band Camera Detailed Mechanical and Servo-MechanicalDescription

With the above overall description in mind, attention is directedprimarily to FIGS. 2A, 2B, and 3-5. The more important mechanicalaspects of the camera will now be described. The optical system 50,including the Cassegrain optical system, spectrum dividing prism 56, andoptical components in the optical paths 58 and 60, are rigidly mountedto a camera housing or superstructure 52. This camera housing 52 takesthe form of a pair of opposed, elongate C-shaped frames extendingtransversely on opposite sides of the roll axis substantially the entirelength of the camera. The C-shaped frame members 52 provide a structurein which to mount the various optical and mechanical components of thecamera, including the end plates 41 and 41A.

The end plate 41 is bolted to the right hand end of the C-shaped frames52, as shown in FIG. 3. The rotor portion of the roll motor 70A is inturn bolted to the end plate 41, thereby coupling the rotational portionof the roll motor 70A to the camera frame 52. The stator portion of theroll motor 70A is fixedly coupled to the aircraft frame or pod via twoL-shaped brackets 39 and the associated passive isolation mounts(conventional, not shown). The left-hand end of the C-shaped frame 52 issimilarly bolted to an end plate 41A, and the rotor portion of the rollmotor 70B is bolted to the end plate 41A, with the stator portion boltedto two L-shaped brackets 39. Two roll motors 70A and 70B areconventional frameless DC torque motors, adapted to mount to the camera36. Two are used in the illustrated embodiment in order supply enoughtorque to rotate the camera housing 52 and all the attached components,but one motor may suffice if it is powerful enough. In the illustratedembodiment, the roll motors 70A and 70B are frameless DC torque motors,adapted to fit to the camera housing, a task within the ability ofpersons skilled in the art. The roll motors are described below infurther detail in conjunction with FIGS. 12-15.

FIG. 3 is a top view of the camera 36, looking towards to the back sideof the primary mirror 80. The Cassegrain objective lens opticalsubassembly 54 includes a primary mirror cell 100 which includes fourmounting flanges 102 with bolt holes 104 for mounting via bolts to thetop flange 106 of the C-shaped frames 52. The Cassegrain optical systemis shown isolated in FIGS. 6-10. In FIGS. 6 and 7, the primary mirror isremoved in order to better illustrate the rest of the structure in theCassegrain optical system.

As is shown best in FIG. 3 and 6, a spider 120 consisting of eight arms122 extends between an inner primary mirror holding ring 110 and anazimuth mirror mounting plate 124 located at the center of the primarymirror 80. The mounting plate 124 incorporates three adjustment screws126 for adjusting the tilt of the azimuth mirror 84. A fiber opticgyroscope 128 is also mounted to the plate 124 and is provided forpurposes of inertial stiffness and stabilization of the Cassegrainoptical system 50. The secondary mirror assembly 113 includes a set ofthree adjustment screws 126A for adjusting and aligning the orientationof the secondary mirror relative to the primary mirror.

The stator portion of the Cassegrain motor 74 is fixed with respect tothe primary mirror cell 100. The rotor portion of the motor 74 ismounted to an annular ring 111 shown in FIG. 10, which is attached tothe inner primary mirror holding ring 110. The secondary mirror 82 isfixed with respect to the primary mirror by means of three arms 112.Thus, the motor 74 rotates both the primary, secondary and azimuthmirrors about axis 75 in the direction of the line of flight in unison.The Cassegrain motor 74 is based on a DC direct drive motor adapted asrequired to the Cassegrain primary mirror holding structure, again atask within the ability of persons skilled in the art.

The rotation of the inner mirror holding ring 110 by the Cassegrainmotor 74 is reduced by a two-to-one reduction tape drive assembly 114,shown best in FIGS. 5, 6 7, 9 and 11. The tape drive assembly 114rotates an azimuth mirror drive shaft 116 that extends from the tapedrive assembly 114 to the azimuth mirror 84. The azimuth tape driveassembly 114 rotates the azimuth mirror drive shaft 116 and thus theazimuth mirror 84 at one half of the rate of rotation of the primary andsecondary mirrors by the Cassegrain motor 74.

The tape drive assembly 114 includes a two-to-one drive housing 150,two-to-one drive couplings 152 and 154, a shaft locking coupling 156,and a pair of stainless steel tapes 158 and 160, the thickness of whichis shown exaggerated in FIG. 11.

Referring to FIG. 12, the roll motor 70A is shown isolated from the restof the camera in a perspective view. FIG. 13 is an elevational view ofthe motor as seen from the other side. FIG. 14 is a cross-sectional viewof the motor 70A. The roll motor 70B is identical to the motor of FIGS.12-14. Additional details concerning the tape drive assembly 114 areconventional and therefore omitted for the sake of brevity.

The motor 70A includes a trunnion 200, a journal 202 and a DC framelessmotor 204. The journal 202 bolts to the plate 41 (FIG. 3) via six boltholes 208. A set of apertures 210 is provided in the face of the journal202 to reduce weight. The sides of the trunnion 200 have opposed,parallel flat surfaces 212 with a series of mounting holes for enablingthe L-shaped mounting brackets 39 to mount to the trunnion 200 in aplurality of different positions. The motor 70A also includes anelectronics module contained in a housing 214. The module includes apower amplifier and associated DC electronic components, which areconventional.

As shown in the cross-sectional view of FIG. 14, and the detail of FIG.15, the motor assembly 70A also includes an annular shim 220, an annularbearing 222, a lock washer 224 and locking nut 226, a trunnion sleeve228, a bearing spacer 230, a bearing insert 232 and a bearing adjustmentplate 234. Additional mechanical features shown in FIGS. 14 and 15 arenot particularly important and therefore are omitted from the presentdiscussion.

Optical System Detailed Description

The optical system design of the subject camera is driven by the need toilluminate a large focal plane image recording medium and by spaceconstraints, namely the total axial length and the total diameter, whichhave to be accounted for in potential aircraft installationapplications. Thus, while the particular optical design described hereinis optimized for a given set of spatial constraints, variation from theillustrated embodiment is considered to be within the scope of theinvention.

The optical system 50 of FIGS. 3 and 4 represents a 50-inch, F/4 opticalsystem designed to operate over an extended spectral region. Theobjective lens module consists of the Cassegrain optical subsystem 54,comprising the primary and secondary mirrors 80 and 82. The azimuthmirror 84 is utilized to redirect the image forming light bundles intothe remainder of the optical system, namely the spectrum dividing prismand the relay lenses and other optical components in the optical paths58 and 60.

Referring now again primarily FIGS. 4, 4A, 4B and 4C, radiation isreflected off the flat azimuth mirror 84 towards a calcium fluoridespectrum-dividing prism 56. An image is formed at a Cassegrain imageplane 130 immediately in front of the prism 56. The spectrum-dividingprism 56 is constructed such that radiation in the visible and near IRband (about 0.5 to about 0.9 microns) passes through the prism 56 intothe visible/near IR optical path 58 while radiation in the MWIR portionof the spectrum (about 3 to about 5 microns) is reflected upwardsthrough a fold prism 132, made from an infrared transmitting material,into the MWIR optical path 60.

In the visible path, the radiation passes through a relay lens assembly62 enclosed in a suitable enclosure 134, a focus element 136 adjusting aset of focus lenses 138, and finally to a shutter 88. An image is formedon the focal plane of the image recording medium 64. The shutter 88opens and closes to control exposure of the visible spectrum imagerecording medium 64. In the illustrated embodiment, the medium 64 is acharge-coupled device E-O detector, comprising an array of pixelsarranged in rows and columns. The array 64 is cooled by athermo-electric cooler 140. The array and thermo-electric cooler areenclosed in their own housing 142, which includes electronics boards 144and a set of heat dissipating cooling fans 146.

In the MWIR path, the light passes through a relay lens assembly 66contained in a suitable housing, through a focus lens assembly 67 and animage is formed at the focal plane of an IR-detecting two dimensionalarray 68. The MWIR sensor comprises the array 68, a cold stop 69, and aninternal filter, all enclosed in a cryogenic dewar 63.

The optical axis of the objective Cassegrain optical subassembly isshown vertical in FIG. 4. This arrangement provides a very compactassembly; if the objective were arranged along a horizontal axis thetotal length required for the system would have been intolerably large.The use of only reflecting components (catoptric) in the objectiveallows the collection of light from a very wide spectral region. Suchimaging would be impractical or impossible with a refracting objectivedesign.

The point of intersection of the visible relay optical axis and theobjective optical axis is an important datum feature of this system. Theazimuth mirror 84 reflecting surface is designed to rotate about an axisthat contains this intersection point. Furthermore, the entireCassegrain objective subassembly 54 is arranged to also rotate aboutthis same axis for forward motion compensation. Rotation of theobjective permits locking onto ground image detail while the camera andaircraft are moving forward. If the azimuth mirror 84 rotates at halfthe angular rate of the objective module with respect to theaircraft/camera frame of reference, the selected ground image iseffectively locked or frozen onto the detectors. Consequently, the imagecan be recorded without blur of relative forward motion between thecamera and the scene.

Presently preferred embodiments of the subject optical system have focallengths of between 50 and 100 inches, and an f/number of between 4.0 and8.0.

FIGS. 16A and 16B are ray diagrams for the visible and MWIR paths of theembodiment of FIG. 4. FIG. 16C is a graph of the diffraction MTF for thevisible path. The MTF curves are wavelength-averaged over the visible/IRspectral range of 500 to 900 nm with system spectral weights. TheCassegrain objective subsystem introduces a central obscuration into thelight forming beams, and therefore reduces the diffraction-limitedperformance limits that can be achieved.

In the interest of completeness of the disclosure of the best modecontemplated for practicing the invention, optical prescription,fabrication and aperture data are set forth below in the followingtables for the embodiment of FIG. 4. Of course, the data set forth inthe tables is by no means limiting of the scope of the invention, anddeparture thereof is expected in other embodiments of the invention.Furthermore, selection and design of the optical components for anygiven implementation of the invention is considered to be a matterwithin the ability of persons skilled in the art of optical design ofaerial reconnaissance cameras, with such additional designs beingconsidered obvious modifications of the illustrated embodiment.

In the tables for the visible and MWIR paths, the numbering of theelements in the left-hand column corresponds to the optical elementsshown in FIGS. 16A and 16B in progression from the entrance aperture tothe detectors.

TABLE 1 VISIBLE PATH PRECRIPTION FABRICATION DATA 30-Aug-00 Modified50″, F/4 VISIBLE PATH ELEMENT  RADIUS OF CURVATURE  APERTURE DIAMETERNUMBER FRONT BACK  THICKNESS  FRONT BACK  GLASS OBJECT   INF  INFINITY*1      C-1     11.3468       APERTURE STOP   C-2 1    A(1)  −11.3468   C-2   REFL 2    A(2)   10.3468   4.8650   REFL   DECENTER(1) 3    INF   0.0000   C-3   REFL       3.3525     −8.2000 4  −14.5000 CX   INF   −0.3000  2.9916  3.0328  CAF2     −0.0200 5   INF  INF   −3.1000  3.0382  3.6125  CAF2      −0.4000 6  INF   5.6254 CXφ −0.6360   3.7191  3.7662  ′F9474/30′      −0.2000 7   INF   INF −0.2500  3.6228  3.5785  ′OG515′      −0.3000 8 −5.4347 CX  33.2980 CX −0.5926  3.4210  3.3057  ′A2334/2′      −0.0200 9 −4.0545 CX  3.6785 CX −0.8920  3.0291  2.7699  ′B2601/1A′ 10 3.6785 CC  −1.4940 CC  −0.2500 2.7699  2.0369  ′12549414′       −0.4633 11 −2.1854 CX  1.4016 CX −0.8744  1.9875  1.8522  ′135662/A′ 12 1.4016 CC  −0.8475 CC  −0.2500 1.8522  1.4085  ′B2651/2′ 13 −0.8475 CX  −1.5553 CC  −0.4845  1.4085 1.3037  ′A2334/2′       −0.3747 14 1.2750 CC  −1.5275 CC  −0.3634 1.3029  1.6318  ′B2650/1′ 15 −1.5275 CX  2.1577 CX  −0.6838  1.6318 1.7456  ′11646656′       −0.0270 16 −5.0620 CX  3.3440 CX  −0.4730 1.8086  1.8207  ′A2334/2′       −0.4251*2 17 −7.5223 CX  19.2220 CX −0.1985  1.6606  1.6522  ′H9420/8′       −0.5725*3 18 −2.5775 CX −1.1718 CC  −0.2500  1.6800  1.5948  ′D1741/4′ 19 −1.1718 CX  2.7961 CX−0.7869  1.5948  1.5304  ′B2601/1A′ 20 2.7961 CC  −2.2230 CC  −0.2500 1.5304  1.4526  ′D1741/4′       −1.7614 21 1.4142 CC  21.3497 CX −0.3802  1.7611  2.1914  ′H9418/35′       −0.0245 22 −3.4440 CX 12.7030 CX  −0.8500  2.4681  2.5450  ′D1741/6′       −0.0500 23 INF  INF  −0.0800  2.5554  2.5603  BK7 Schott   IMAGE DISTANCE =  −0.9162IMAGE   INF   2.6440 NOTES Positive radius indicates the center ofcurvature is to the right Negative radius indicates the center ofcurvature is to the left Dimensions are given in inches Thickness isaxial distance to next surface Image diameter shown above is a paraxialvalue, it is not a ray traced value Other glass suppliers can be used iftheir materials are functionally equivalent to the extent needed by thedesign; contact the designer for approval of substitutions. APERTUREDATA       DIAMETER  DECENTER APERTURE   SHAPE    X  Y  X  Y  ROTATIONC-1  CIRCLE   13:168   CIRCLE  (OBSC)  4.800  4.800 C-2  CIRCLE   (OBSC) 4.800  4.800   CIRCLE   12.600   12.600 C-3  RECTANGLE   3.500  4.000 0.000  0.100  0.0 ASPHERIC CONSTANTS   2  (CURV)Y    4   6   8   10 Z =------------------------------+(A)Y   + (B)Y   + (C)Y   + (D)Y 2 2 1/21 + (1 − (1 + K) (CURV) Y ASPHERIC   CURV   K   A   B   C   D A(1)  −0.02881267   −1.000000 A(2)   −0.05583473   −3.928273 DECENTERINGCONSTANTS DECENTER  X   Y   Z   ALPHA   BETA   GAMMA D(1)  0.0000 0.0000  0.0000  35.0000  0.0000  0.0000 (BEND)

TABLE 2 MWIR PRESCRIPTION FABRICATION DATA Modified 50, F/4 MWIR LENS 2Fit to POD Testplates ELEMENT RADIUS OF CURVATURE   APERTURE DIAMETERNUMBER FRONT BACK THICKNESS  FRONT  BACK  GLASS OBJECT   INF  INFINITY*1    C-1     11.3468 1   A(1) (Paraboloid) −11.3468   C-2  REFL 2   A(2)(Ellipsoid)  10.3468   44000  REFL   DECENTER(1) 3   INF.  −8.2000  3.9482  REPL  (Azimuth Mirror) 4  31 14.5000 CX INF −0.3000 3.2080 3.2480  CAF2 (Field Lens)      −0.0200 5 INF  INF −1.5500  C-3  C-4 CAF2  DECENTER(2)     C-4  REFL  INF INF  INF  1.5500  C-4  C-5  CAF2   0.6250 6 INF  INF  2.4000  C-6  C-7  SILICON  DECENTER(3)  INF   C-7 REFL  INF  INF  −2.5000  C-7  C-8  SILICON     0.1000 7  −5.2363 CX −11.8133 CC −0.6000 4.3400  4.1700  SILICON     −1.2602 8  7.0285 CC  A(3) −0.3500  3.1100  3.1800  GERMMW    −0.4399 9  3.5768 CC  10.0404 CX −0.3500  3.2000  3.7400  ZNS    −0.0638 10 27.0245 CC  5.1234 CX −0.6000  4.1200  4.2400  SILICON    −1.9286 11   A(4)  10.1487 CX −0.4000  3.8400  3.8200  ZNS    −0.9223 12  −2.2473 CX  −2.5307 CC −0.3500  2.6700  2.4200  ZNSE     −0.2702*2 13  −2.0677 CX  −1.4306 −0.5000  2.1000  1.5600  ZNS     −0.6180*3 14  INF   INF  −0.1180 1.1400  1.1400  SILICON     −0.2290     APERTURE STOP   C-9  (ColdStop)     −1.2070 15  INF   INF  −0.0400  C-10  C-11  SILICON  IMAGEDISTANCE = −2.13000 IMAGE   INF   2.8293 NOTES Positive radius indicatesthe center of curvature is to the right Negative radius indicates thecenter of curvature is to the left Dimensions are given in inchesThicknes is axial distance to next surface Image diameter shown above isa paraxial value it is not a ray traced value Other glass suppliers canbe used if their materials are functionally equivalent to the oxtentneeded by the design; contact the designer for approval of substitutionsAPPERTURE DATA    DIAMETER   DECENTER APERTURE  SHAPE  X  Y  X  Y ROTATION C-1   CIRCLE   12.633    CIRCLE  (OBSC)  4.800  4.800 C-2  CIRCLE   12.500   12.500    CIRCLE  (OBSC)  4.800  4.800 C-3 RECTANGLE   2.900  2.900  (CaF2 Prism - Entrance Face) C-4  RECTANGLE  2.900  4.101  (CaF2 Prism - Splitter Face) C-5  RECTANGLE   2.900 2.900  (CaF2 Prism-Exit Face) C-6  RECTANGLE   3.050  3.050 (SiliconPrism-Entrance Face) C-7  RECTANGLE   3.300  5.640  0.000 −0.149  0.0C-8  RECTANGLE   3.410  3.410 (Siiicon Prism-Exit Face) C-9  CIRCLE  0.844  0.844  (Cold Stop)   CIRCLE  (OBSC)  0.320  0.320  (OccultingDisk) C-10  RECTANGLE   1.280  1.280 C-11  RECTANGLE   1.280  1.280ASPHERIC CONSTANTS     2   (CURV)Y  4  6  8  10 Z =--------------------+ (A) Y  + (B)Y  + (C)Y  + (D)Y    2 2 1/2  1 + (1 −(1 + K(CURV)Y) ASPHERIC  CURV  K  A  D  C  D A(1)  −0.02381267  −1.000000 A(2)  −0.05583473   −3 .928273 A(3) −0.02571678 0.000000−3.39014E-03 3.98460E-04 −2.25842E-05 0.00000E+00 A(4) −0.020189300.000000 6.40826E-04 8.94387E-05 1.85905E-05 2.93941E-06

Electronics System

The electronics for the camera 36 of FIGS. 1 and 3 is shown in blockdiagram form in FIG. 17. The electronics includes an image processingunit (IPU) 401 which contains the master control computer 34 of FIG. 1.The master control computer 34 supplies control signals along aconductor 400 to a camera body and stabilization electronics module,represented by block 402. The camera body and stabilization electronics402 basically includes digital signal processing cards that providecommands to the roll motors 70A and 70B and the Cassegrain or azimuthmotor 74 of FIGS. 3, 5 and 6, and receive signals from the stabilizationsystem consisting of the azimuth fiber optic gyroscope 128 mounted onthe azimuth mirror and a roll fiber optic gyroscope (not shown) mountedon the camera housing 52. The camera body electronics 402 also receivescurrent roll angle and roll rate data from resolvers in the roll motors70A and 70B, and from the roll gyroscope, and supplies the rollinformation to the camera control computer.

The camera control computer 34 also generates control signals, such asstart, stop, and counter values, and supplies them via conductor 406 toan IR sensor module (IRSM) 408 and a Visible Sensor Module 410. The IRSM408 includes a cryogenic dewar or cooler 63, the IR detector 68 (FIG. 4)and associated readout circuitry, and electronic circuitry shown in FIG.19 and described subsequently for transferring charge through the IRarray to achieve roll motion compensation. Pixel informationrepresenting IR imagery is read out of the array 68, digitized, and sentalong a conductor 412 to the IPU 401. In an alternative embodiment, theelectronic circuitry shown in FIG. 19 could be incorporated into thecamera body electronics 402 or in the Image Processing Unit 401.

The visible sensor module 410 includes a mechanical shutter 88, avisible spectrum electro-optical detector 64 (FIG. 4) and associatereadout registers, and electronic circuitry described in FIG. 19 anddescribed subsequently for transferring charge through the visiblespectrum detector 64 to achieve roll motion compensation. Pixelinformation representing visible spectrum imagery is read out of thedetector 64, digitized, and sent along a conductor 414 to the IPU 401.

Visible and IR imagery supplied by the Visible Sensor Module and the IRSensor Module is received by a dual band input module 420 and suppliedto an image processor 422 for purposes of contrast adjustment,filtering, radiometric correction, etc. Typically, images generated bythe arrays 64 and 68 are either stored for later retrieval or downlinkedto a ground station. In the illustrated embodiment, the imagery iscompressed by a data compression module 424, supplied to an outputformatter 426 and sent along a conductor 428 to a digital recordingmodule 430 for recording of the imagery on board the aircraft.

Aircraft inertial navigation system data such as aircraft velocity,height, aircraft attitude angles, and possibly other information, isobtained from an aircraft 1553 bus, represented by conductor 432.Operator inputs such as start, stop and roll angle commands from amanual cockpit or camera console or control panel, can also be suppliedalong the conductor 432 or by an optional control conductor 434. The INSand operator commands are processed in an INS interface circuit 436 andsupplied to the camera control computer 34 and used in the algorithmsdescribed above. The camera control computer also has a non-volatilememory (not shown) storing fixed parameters or constants that are usedin generating the roll motion compensation commands, such as the pixelpitch, array size, master clock rate, and optical system focal length.

The image processor 422 and a graphics module 438 are used to generatethumbnail imagery and supply the imagery to an RS-170 output 440 forviewing in near real time by the operator or pilot on board thereconnaissance vehicle, or for downloading to the ground station. Otherformat options for the thumbnail imagery are also possible.

Aircraft power is supplied to a power conversion unit 442, whichfilters, converts and distributes it to two power modules 444. The powermodules 444 supply the required AC or DC voltages to the variouselectronic components in the camera 36.

An RS-232 diagnostic port 446 is provided in the IPU 401 for remoteprovisioning, diagnostics, and software downloads or upgrades ordebugging by a technician. The port 446 provides an interface to themaster control computer 34, and the other modules in the IPU 401 andallows the technician to access these units with a general purposecomputer. Changes to fixed parameters stored in non-volatile memory,such as a change in the focal length of the camera, are also made viathe port 446.

Except as noted herein and elsewhere in this document, the individualmodules and components in the electronics are considered to beconventional and therefore can be readily derived be persons skilled inthe art. Accordingly, a detailed discussion of the modules per se isomitted from the present discussion.

Roll Motion Compensation

Referring now to FIG. 18, a presently preferred implementation of rollmotion compensation in an electro-optical area array detector will nowbe described. The visible/near IR E-O detector 64 is shown in a planview. The detector consists of an array of pixel elements 300 arrangedin a plurality of rows and columns, with the column direction chosen tobe across the line of flight and the row direction in the direction offlight. The array 64 can be any suitable imaging detector including acharge-coupled device, and preferably will comprise at least 5,000pixels in the row direction and at least 5,000 pixels in the columndirection. The illustrated embodiment consists of 5040×5040 pixels, witha 0.010 mm×0.010 mm pixel pitch and a 50.4 mm×50.4 mm array size. Thereader is directed to the Lareau et al. U.S. Pat. No. 5,155,597 patentfor a suitable detector, however the array need not be organized intocolumn groups as described in the '597 patent and could be configured asa single column group, all columns of pixels clocked at the same rate.

The architecture for the array is not critical, but a full frame imager,as opposed to an interline transfer architecture, is presentlypreferred. The imager can use either a mechanical shutter or anelectronic shutter to expose the array.

The roll motion caused by camera roll motors 70A and 70B produces animage motion indicated by the arrows 302 in the plane of the array 64.The roll motion is in the cross-line of flight direction and the imagevelocity v is nearly constant throughout the array. The velocity v isequal to the product of the optical system focal length f multiplied bythe rate of rotation ω. Since f is fixed (and the value stored in memoryfor the camera control computer), and the rate of rotation is known byvirtue of outputs of the fiber-optic gyroscope 128 or from resolvers inthe roll motors, the velocity of the image due to roll can be preciselydetermined for every exposure. The velocity can be expressed in terms ofmm/sec, in terms of rows of pixels per second, or in terms of thefraction of a second it takes for a point in the image to move from onerow of pixels to the adjacent row, given the known pixel pitch. Thepixel information (i.e., stored charge) in the individual pixels 300 istransferred row by row throughout the entire array 64 at the same rateand in the same direction of image motion during the exposure time,thereby avoiding image smear due to the roll motion.

To accomplish this, and with reference to FIG. 19, the cameraelectronics includes a counter and clock driver circuit 304 (one foreach detector 64, 68) which generates voltage pulses and supplies themto a set of three phase conductors 308 which are coupled to each row ofthe array. One cycle of three-phase clocking effectuates a transfer ofcharge from one row to the adjacent row. A master clock 306 generatesclock signals at a master clock frequency and supplies them to a counter310. The camera control computer calculates a counter value whichdetermines the number the counter 310 is supposed to count to at theknown master clock rate to time the transfer of charge from one row toanother in synchronism with the movement of the image by one row ofpixels (0.010 mm). The master computer 34 supplies the counter value tothe counter 310, along with a start and stop commands.

At the moment the array 64 is exposed to the scene, the counter 310starts counting at the clock rate up to the counter value. When thecounter value is reached, a trigger signal is sent to a clock driver312. The clock driver 312 initiates one cycle of three phase clocking onconductors 308, causing the pixel information from row 1 to betransferred to row 2, from row 2 to row 3, etc. When the counter valueis reached, the counter 310 resets itself and immediately beginscounting again to the counter value, another cycle of clocking istriggered, and the process repeats continuously while the array isexposed and charge is integrated in the detectors. At the end of theexposure period, a stop signal is sent to the counter 310. The pixelinformation in the array 64 is read out of the array into read-outregisters at the bottom of the array (not shown), converted into digitalform, and either stored locally on a digital recording medium for lateruse or transmitted to a remote location such as a base station.

The process described for array 64 is essentially how the IR detectoroperates as well for accomplishing roll motion compensation. Inalternative embodiments, the image motion compensation could beperformed in other readout structures depending on the architecture forthe array. The IR detector could be sensitive to radiation in the ShortWavelength Infra-Red (SWIR) band (1.0 to 2.5 microns), Mid-Wavelength IR(MWIR) band (3.0 to 5.0 microns) or Long Wavelength IR (LWIR) band (8.0to 14.0 microns). In such an array, the output of the eachphotosensitive photodiode detector is coupled to a charge storagedevice, such as a capacitor or CCD structure, and the charge is shiftedfrom one charge storage device to the adjacent charge storage device insynchronism with the image velocity while charge is being integrated inthe charge storage devices.

The process of roll motion compensation can be more finely tuned byderiving the rate of rotation (ω) used in the algorithm from the actualinertial rate sensed by a fiber optic gyroscope mounted to the camerahousing or frame. Such a gyroscope can count with a resolution of 1microradian or better. The gyroscope generates a signal that is suppliedto a DSP card in the camera control electronics module 402 (FIG. 17). Asignal could also be constructed for imaging array clocking purposes inthe form of a pulse train which the imaging array clock generator couldphase lock to. By doing this, any rate inaccuracy or stabilizationshortcomings associate with the roll motion could be overcome. The rollmotion compensation becomes, in effect, a fine stabilization systemwhich removes the residual error from the more coarse electro-mechanicalstabilization system. Having a fine system, based on a closed loopfeedback from the roll fiber optic gyroscope, would allow for a largerrange of roll motion without image degradation.

The above-described roll motion compensation will produce some minoredge effects at the bottom of the array, which are typically ignoredsince they are a very small fraction of the image generated by thearray.

Other Embodiments

As noted above, the principles of roll framing and forward motioncompensation described above are applicable to a camera that images in asingle band of the electromagnetic spectrum. In such an embodiment, thespectrum dividing prism would not been needed and the objective opticalsubassembly (Cassegrain or otherwise) would direct the radiation in theband of interest to a single optical path having a photo-sensitive imagerecording medium placed herein. The spectrum dividing prism and secondoptical channel are not needed. Otherwise, the operation of the camerain roll framing and spot modes of operation would be the same asdescribed above.

As another alternative embodiment, three or more detectors could imagethe three or more bands of the electromagnetic spectrum simultaneously.In such an embodiment, an additional spectrum separating prism would beplaced in either the visible or IR paths to further subdivide theincident radiation into the desired bands and direct such radiation intoadditional optical paths, each with its own photo-sensitive imagerecording medium. As an example, the visible/near IR band could bedivided into a sub 700 nanometer band and a 700 to 1000 nanometer band,each associated with a distinct optical path and associated imageforming and focusing lenses and an image recording medium. Meanwhile,the IR portion of the spectrum could be similarly divided into twoseparate bands, such as SWIR, MWIR, and/or LWIR bands, and each bandassociated with a distinct optical path and associated image forming andfocusing lenses and an image recording medium. Obviously, in such anembodiment the arrangement of optical components in the camera housingwill be different from the illustrated embodiment due to the additionalspectrum dividing prisms, additional optical paths and opticalcomponents, and additional detectors. However, persons skilled in theart will be able to make such a modification from the illustratedembodiment using routine skill.

As yet an another possible embodiment, the camera may be designed forhyperspectral imaging. In such an embodiment, one of the optical pathsmay be devoted to visible spectrum imaging, while the other path isfitted with a spectroradiometer, an imaging spectrometer, orspectrograph to divide the incident radiation into a large number ofsub-bands in the spectrum, such as 50 of such sub-bands. Each sub-bandof radiation in the scene is imaged by the hyperspectral imaging array.

As yet another alternative, the camera could be mounted transverse tothe roll axis of the aircraft. Such a camera could be used for dualspectrum, full framing imaging in a forward oblique mode, either in aspot mode of operation or in a mode in which overlapping frames ofimages are generated in a forward oblique orientation.

As yet another alternative embodiment, the smooth roll motion and rollmotion compensation feature could be adapted to a step framing camera,such as the KS-127A camera or the step frame camera of the Lareau et al.U.S. Pat. No. 5,668,593. In this embodiment, the roll motors are coupledto the step frame scan head assembly, and continuously rotate the scanhead about the roll axis in a smooth, continuous fashion. The detectorarray and associated relay and focusing optical elements remainstationary with respect to the aircraft. The image acquired by the scanhead assembly would need to be derotated with a pechan prism, K mirroror other suitable element, as described in the '593 patent. Roll motioncompensation would be performed electronically in the array, asdescribed at length above.

As a variation on the above embodiment, the roll motors are coupled tothe step frame scan head and continuously rotate the step frame scanhead assembly, while the image derotation is achieved by rotation of theimaging array in synchronism with the rotation of the scan headassembly. Roll motion compensation is achieved by transferring pixelinformation in the array at substantially the same rate as the rate ofimage motion due to scan head rotation.

Less preferred embodiments of the invention include other types ofoptical arrangements. While the catoptric Cassegrain optical system isthe preferred embodiment, refractive optical systems, catadioptricoptical systems, and still other types of optical arrangements may beused, for example where only single spectrum imaging is performed, wherespace requirements are not so important, or when other considerationsdictate that a different type of optical arrangement for the objectivelens is suitable. In such embodiments, the optical subassemblycomprising the objective lens would be rotated in the direction offlight to accomplish forward motion compensation as described above,while the entire camera housing including the objective lens is rotatedabout an axis to thereby generate sweeping coverage of the field ofinterest, either about the roll axis or about an axis perpendicular tothe roll axis.

Presently preferred and alternative embodiments of the invention havebeen described with particularity. Considerable variation from thedisclosed embodiments is possible without departure from the spirit andscope of the invention. For example, the type and structure of the imagerecording medium is not critical. The details of the optical design, themechanical system and the electronics may vary from the illustrated,presently preferred embodiments. This true scope and spirit is to bedetermined by the appended claims, interpreted in light of theforegoing.

We claim:
 1. A dual band framing aerial reconnaissance camera forinstallation in an aerial reconnaissance vehicle, comprising; (a) anoptical system incorporated into a camera housing, comprising: (1) anobjective optical subassembly for receiving incident radiation from ascene external of said vehicle; (2) a spectrum dividing elementreceiving radiation from said objective optical subassembly, saidelement directing radiation in a first band of the electromagneticspectrum into a first optical path and directing radiation in a secondband of the electromagnetic spectrum into a second optical pathdifferent from said first optical path; (3) a first two-dimensionalimage recording medium in said first optical path for generating framesof imagery in said first band of the electromagnetic spectrum; and (4) asecond two-dimensional image recording medium in said second opticalpath for generating frames of imagery in said second band of theelectromagnetic spectrum; (b) a first motor system coupled to saidcamera housing rotating said camera about a first axis, said camerahousing installed in said aerial reconnaissance vehicle such that saidfirst axis of rotation is parallel to the roll axis of said aerialreconnaissance vehicle, wherein said image recording media are exposedto said scene to generate frames of imagery as said first motor systemrotates said camera in a continuous fashion about said first axis, saidfirst and second image recording media having a means for compensatingfor image motion due to said rotation of said camera; and (c) a secondmotor system coupled to said objective optical subassembly, said secondmotor system rotating said objective optical subassembly about a secondaxis in the direction of forward motion of said vehicle to compensatefor forward motion of said aerial reconnaissance vehicle.
 2. The cameraof claim 1, wherein said first and second image recording media comprisetwo dimensional area array electro-optical detectors.
 3. The camera ofclaim 2, wherein one of said electro-optical detectors is sensitive toradiation in the ultraviolet (UV) portion of the electromagneticspectrum and wherein the other of said electro-optical detectors issensitive to radiation in the infrared portion of the electromagneticspectrum.
 4. The camera of claim 3, wherein the detector sensitive toradiation in the infrared portion of the electromagnetic spectrum issensitive to radiation having a wavelength of between 1.0 to 2.5microns.
 5. The camera as claimed in claim 2, wherein saidelectro-optical detectors comprise an array of pixel elements arrangedin a plurality of rows and columns, and wherein said means forcompensation for roll motion of said camera housing comprises electroniccircuitry for transferring pixel information in said electro-opticaldetectors from row to adjacent row at a pixel information transfer ratesubstantially equal to the rate of image motion in the plane of saidelectro-optical detectors due to roll of said camera housing.
 6. Thecamera as claimed in claim 5, wherein said camera further comprises acamera control computer calculating said pixel information transfer ratefrom system inputs comprising f, the focal length of said opticalsystem, and ω, the rate of rotation of said camera housing about saidroll axis.
 7. The camera as claimed in claim 1, wherein at least one ofsaid image-recording media comprises photosensitive film.
 8. The cameraas claimed in claim 7, wherein said means for compensating for rollmotion of said camera housing comprises a mechanism for moving said filmat a rate substantially equal to the rate of image motion in the planeof said film due to roll of said camera housing.
 9. The camera asclaimed in claim 8, wherein said camera further comprises a cameracontrol computer calculating said rate of movement of said film fromsystem inputs comprising f, the focal length of said optical system, andω, the rate of rotation of said camera housing about said roll axis. 10.The camera as claimed in claim 1, wherein said objective opticalsubassembly comprises a catoptric Cassegrain optical system which formsan image at a Cassegrain image plane, and wherein said catoptricCassegrain optical system comprises a primary mirror, a secondary mirrorrigidly coupled to said primary mirror, and a flat azimuth mirrorlocated in the optical path between the secondary mirror and theCassegrain image plane.
 11. The camera as claimed in claim 10, whereinsaid second motor system comprises a Cassegrain motor coupled to saidprimary mirror, said secondary mirror and said azimuth mirror, andwherein to compensate for forward motion of said vehicle said Cassegrainmotor rotates said primary and secondary mirrors in the flight directionat a rate equal to V/R where V is the velocity of aerial reconnaissancevehicle and R is either the range to the scene of interest or anapproximation of said range, and rotates said azimuth mirror at rateequal to ½ (V/R) in the same direction as the rotation of said primaryand secondary mirrors due to said Cassegrain motor.
 12. The camera asclaimed in claim 11, wherein the value of R is derived from the heightof said vehicle above the earth and a camera depression angle below ahorizontal reference frame.
 13. The camera as claimed in claim 11,wherein the value of R is derived using a range finder on board thevehicle.
 14. The camera as claimed in claim 11, wherein the value of Ris derived from a global positioning system.
 15. The camera as claimedin claim 11, wherein the value of R is derived from processingsuccessive frames of imagery from at least one of said image-recordingmedia.
 16. The camera of claim 11, wherein said secondary mirror iscentrally located in the entrance aperture of said catoptric Cassegrainoptical system.
 17. The camera as claimed in claim 1, wherein saidoptical system has an overall focal length of between 50 and 100 inchesand a f/number of between 4.0 and 8.0.
 18. The camera as claimed inclaim 1, further comprising a camera control computer operative of saidimage recording media and said first and second motor systems togenerate a series of overlapping frames of imagery across the line offlight of said vehicle as said camera housing is rotated about saidfirst axis, and wherein each of said overlapping frames of imagery isrecorded in two different portions of said electromagnetic spectrum bysaid first and second image recording media.
 19. The camera as claimedin claim 18, wherein said camera further comprises a spot mode ofoperation, said camera control computer in said spot mode of operationoperative of said first and second motor systems to orient saidCassegrain optical system at a selected camera depression angle below ahorizontal reference plane and fore/aft azimuth angle, and operative ofsaid first and second image recording media to generate first and secondframes of imagery of said scene at said camera depression angle andfore/aft azimuth angle.
 20. The camera as claimed in claim 19, whereinsaid objective optical subassembly comprises a Cassegrain optical systemforms an image at a Cassegrain image plane, and wherein said Cassegrainoptical system comprises a primary mirror, a secondary mirror rigidlycoupled to said primary mirror, and a flat azimuth mirror located in theoptical path between the secondary mirror and the Cassegrain imageplane.
 21. The camera as claimed in claim 20, wherein said second motorsystem comprises a Cassegrain motor coupled to said primary mirror andsaid azimuth mirror, and wherein to compensate for forward motion ofsaid vehicle said Cassegrain motor rotates said primary and secondarymirrors in the flight direction at a rate equal to V/R where V is thevelocity of aerial reconnaissance vehicle and R is either the range tothe scene of interest or an approximation of said range, and whereinsaid azimuth mirror is rotated at rate equal to ½ (V/R) in the samedirection as the rotation of said primary and secondary mirrors due tosaid Cassegrain motor.
 22. The camera as claimed in claim 21, whereinthe value of R is derived from the height of said vehicle above theearth and a camera depression angle below a horizontal reference frame.23. The camera as claimed in claim 21, wherein the value of R is derivedusing a range finder on board the vehicle.
 24. The camera as claimed inclaim 21, wherein the value of R is derived from a global positioningsystem.
 25. The camera as claimed in claim 21, wherein the value of R isderived from a processing of successive images generated by at least oneof said image-recording media.
 26. The camera as claimed in claim 20wherein said secondary mirror is centrally located in the entranceaperture of said Cassegrain optical system.
 27. The camera of claim 2,wherein one of said electro-optical detectors is sensitive to radiationin the visible portion of the electromagnetic spectrum and wherein theother of said electro-optical detectors is sensitive to radiation in theinfrared portion of the electromagnetic spectrum.
 28. The camera ofclaim 3, wherein said electro-optical detector sensitive to radiation inthe infrared portion of the electromagnetic spectrum is sensitive toradiation having a wavelength of between 3.0 and 5.0 microns.
 29. Thecamera of claim 3, wherein said electro-optical detector sensitive toradiation in the infrared portion of the electromagnetic spectrum issensitive to radiation having a wavelength of between 8.0 and 14.0microns.
 30. The camera of claim 27 wherein said electro-opticaldetector sensitive to radiation in the infrared portion of theelectromagnetic spectrum is sensitive to radiation having a wavelengthof between 1.0 and 2.5 microns.
 31. The camera of claim 27, wherein saidelectro-optical detector sensitive to radiation in the infrared portionof the electromagnetic spectrum is sensitive to radiation having awavelength of between 3.0 and 5.0 microns.
 32. The camera of claim 27,wherein said electro-optical detector sensitive to radiation in theinfrared portion of the electromagnetic spectrum is sensitive toradiation having a wavelength of between 8.0 and 14.0 microns.
 33. Aframing aerial reconnaissance camera for installation in an aerialreconnaissance vehicle, comprising; (a) an optical system incorporatedinto a camera housing, comprising: (1) an objective optical subassemblyreceiving incident radiation from a scene external of said vehicle; (2)an optical channel receiving radiation from said objective opticalsubassembly, and (3) a two-dimensional image recording medium in saidoptical channel for generating frames of imagery in a band of theelectromagnetic spectrum, said optical channel including one or moreoptical elements focusing radiation from said scene on saidtwo-dimensional image recording medium; (b) a first motor system coupledto said camera housing rotating said camera about a first axis, saidcamera housing installed in said aerial reconnaissance vehicle such thatsaid first axis of rotation is parallel to the roll axis of said aerialreconnaissance vehicle, wherein said image recording medium is exposedto said scene to generate frames of imagery as said first motor systemrotates said camera in a continuous fashion about said first axis, saidimage recording medium having a means for compensating for image motiondue to said rotation of said camera housing; and (c) a second motorsystem coupled to said objective optical subassembly, said second motorsystem rotating said objective optical subassembly about a second axisin the direction of forward motion of said vehicle about an axis tocompensate for forward motion of said aerial reconnaissance vehicle. 34.The camera of claim 33, wherein said image recording medium comprises atwo-dimensional area array electro-optical detector.
 35. The camera ofclaim 34, wherein said electro-optical detector is sensitive toradiation in the ultraviolet portion of the electromagnetic spectrum.36. The camera of claim 34, wherein said electro-optical detector issensitive to radiation in the infrared portion of the electromagneticspectrum.
 37. The camera of claim 36, wherein said electro-opticaldetector is sensitive to radiation having a wavelength of between 1.0 to2.5 microns.
 38. The camera as claimed in claim 34, whereinelectro-optical detector comprises an array of pixel elements arrangedin a plurality of rows and columns, and wherein said means forcompensation for roll motion of said camera housing comprises electroniccircuitry for transferring pixel information in said electro-opticaldetector from row to adjacent row at a pixel information transfer ratesubstantially equal to the rate of image motion in the plane of saidelectro-optical detector due to roll of said camera housing.
 39. Thecamera as claimed in claim 38, wherein said camera further comprises acamera control computer calculating said pixel information transfer ratefrom system inputs comprising f, the focal length of said opticalsystem, and ω, the rate of rotation of said camera housing about saidroll axis.
 40. The camera as claimed in claim 33, wherein said imagerecording medium comprises photosensitive film.
 41. The camera asclaimed in claim 40, wherein said means for compensating for roll motionof said camera housing comprises a mechanism for moving said film at arate substantially equal to the rate of image motion in the plane ofsaid film due to roll of said camera housing.
 42. The camera as claimedin claim 41, wherein said camera further comprises a camera controlcomputer calculating said rate of movement of said film from systeminputs comprising f, the focal length of said optical system, and ω, therate of rotation of said camera housing about said roll axis.
 43. Thecamera as claimed in claim 33, wherein said optical has an overall focallength of between 50 and 100 inches and a f/number of between 4.0 and8.0.
 44. The camera as claimed in claim 33, further comprising a cameracontrol computer operative of said image recording medium and said firstand second motor systems to generate a series of overlapping frames ofimagery across the line of flight of said vehicle as said camera housingis rotated about said first axis.
 45. The camera as claimed in claim 44,wherein said camera further comprises a spot mode of operation, saidcamera control computer in said spot mode of operation operative of saidfirst motor systems to orient said objective optical subassembly at aselected camera depression angle about the roll axis of said vehicle andfore/aft azimuth angle, and operative of said first and second imagerecording media to generate first and second frames of imagery of saidscene at said camera depression angle and fore/aft azimuth angle. 46.The camera of claim 34, wherein said electro-optical detector issensitive to radiation in the visible portion of the electromagneticspectrum.
 47. The camera of claim 36, wherein said electro-opticaldetector is sensitive to radiation having a wavelength of between 3.0and 5.0 microns.
 48. The camera of claim 36, wherein saidelectro-optical detector is sensitive to radiation having a wavelengthof between 8.0 and 14.0 microns.
 49. A framing aerial reconnaissancecamera for installation in an aerial reconnaissance vehicle, comprising;(a) an optical system incorporated into a camera housing, comprising:(1) a Cassegrain optical system for receiving incident radiation from ascene external of said vehicle; (2) an optical channel receivingradiation from said Cassegrain optical system, and (3) a two-dimensionalelectro-optical detector in said optical channel for generating framesof imagery in a band of the electromagnetic spectrum, said opticalchannel including one or more optical elements focusing radiation fromsaid scene on said two-dimensional electro-optical detector; (b) a firstmotor system coupled to said camera housing rotating said camera about afirst axis, said camera housing installed in said aerial reconnaissancevehicle such that said first axis of rotation is parallel to the rollaxis of said aerial reconnaissance vehicle, wherein said detector isexposed to said scene to generate frames of imagery as said first motorsystem rotates said camera in a continuous fashion about said firstaxis, and (c) a second motor system coupled to said Cassegrain opticalsystem, said second motor system rotating said Cassegrain optical systemabout a second axis in the direction of forward motion of said vehicleabout an axis to compensate for forward motion of said aerialreconnaissance vehicle, wherein said electro-optical detector comprisesan array of pixel elements arranged in a plurality of rows and columns,and wherein said detector further comprises electronic circuitry coupledto said array transferring pixel information in array from row toadjacent row at a pixel information transfer rate substantially equal tothe rate of image motion in the plane of said electro-optical detectorsdue to roll of said camera; said electronic circuitry and said secondmotor system operative to achieve roll motion compensation and forwardmotion compensation simultaneously in said array to thereby enable highresolution images to be obtained from said electro-optical detector. 50.The camera as claimed in claim 49, wherein said camera further comprisesa camera control computer calculating said pixel information transferrate from system inputs comprising f, the focal length of said opticalsystem, and so, the rate of rotation of said camera housing about saidroll axis.
 51. A method of generating frames of imagery of a scene ofinterest with an aerial reconnaissance camera in two different bands ofthe electromagnetic spectrum simultaneously, comprising the steps of:providing two photosensitive electro-optical detectors in said camera,each of said detectors comprising an array of pixel elements arranged ina plurality of rows and columns; rotating said camera in a continuousfashion about a roll axis either coincident with or parallel to a rollaxis of an aerial reconnaissance vehicle carrying said camera; whilerotating said camera, simultaneously exposing said electro-opticaldetectors to a scene of interest in a series of exposures; whilerotating said camera and while exposing said electro-optical detectorsto said scene, rotating an optical system providing an objective lensfor said camera in the direction of forward motion of said vehicle at apredetermined rate to cancel out image motion due to forward motion ofsaid vehicle; and while said electro-optical detectors are being exposedto said scene, moving pixel information in said arrays at a rate and ina direction substantially equal to the rate of image motion due torotation of said camera about said roll axis to thereby preserveresolution of an image generated by said detectors.